1. Field of the Invention
This invention relates to an improved method for preparing a polyamide. More particularly, this invention relates to the method using a specific condensing agent.
2. Description of the Prior Art
As particularly described in "Condensation Polymers by Interfacial and Solution Methods" (Morgan; Interscience, New York, 1965), Morgan and his school of E. I. Du Pont de Nemours & Company (U.S.A.) made a systematic study on interfacial condensation polymerization and low temperature solution polymerization using an acid chloride for preparing polyamides, and established the simple methods for preparing polyamides at room temperature in 1958. The methods have been widely used for preparing polyamides, and industrialized to produce polyamides, such as Nomex.RTM. and Kevlar.RTM..
Further, various studies for synthesizing polyamides under mild and neutral conditions have been made, and some of the methods, such as a process through an active ester intermediate, have been reported [see, for example, Ueda et al.; Journal of Polymer Science, Polymer Chemistry Edition Vol. 23, 1607-1613 (1985)].
However, Morgan's methods are subject to problems, such as generation of toxic gas and corrosion of apparatus, because Morgan's methods are carried out under strong acidic conditions. Further, disposal of a large amount of produced acidic waste water is also a big problem. Moreover, the products by Morgan's methods are usually contaminated with several hundred ppm of chlorine because of a certain side reaction, and it is difficult to remove the chlorine from the product. Furthermore, polyamides produced by Morgan's methods contain chloride ions as impurities, and it is very difficult to decrease them to several ppm or less by an ordinary process.
Polyamides and polymers produced from the polyamides, such as polyimides and polyamide-imides, have been widely used as materials for electric or electronic articles, such as insulators flexible print circuit boards and molding materials. Further, these polymers are newly used for coating materials for semiconductor interlayer insulation materials for LSI (large scale integrated) circuits and thermal resistant films. In these applications, even a trace amount of chloride ion brings about a serious problem of corrosion or lowering of the capability of the device.
Methods proposed by Ueda et al. also have problems; complicated operations are required to isolate the intermediate. Further, since the intermediate is produced under strongly acidic conditions, the methods have not yet been industrialized.
Accordingly, a new process which can be carried out under mild and neutral conditions and can produce polyamides having no undesirable impurities, such as chloride ions, is desired.